420mpa grade hot-rolled strip for building structures resistant to corrosion from sea spray and method of manufacturing the same

Abstract

420MPa grade hot-rolled strip for building structure resisting splash zone corrosion of sea wave and its manufacturing method, the hot-rolled strip includes base layer, corrosion resistant layer and interface transition layer between base layer and corrosion resistant layer; the base layer component mass percentage is: C 0.03~0.15%, Si 0.15~0.35%, Mn 1.0~1.5%, P 0.0005~0.003%, S 0.0005~0.01%, Cr 0.08~0.45%, Ni 0.08~0.60%, Cu 0.05~0.2%, Al 0.02~0.05%, Ti 0.009~0.016%, Nb 0.03~0.06%, N 0.0005~0.005%, V 0.08~0.25%, the balance includes Fe and inevitable impurities; the corrosion resistant layer uses industrial pure titanium.The yield strength of the hot-rolled strip for building structure of the application is greater than or equal to 420MPa, the tensile strength is greater than or equal to 570MPa, the yield strength ratio is less than or equal to 0.79, the impact energy at-40 DEG C is greater than or equal to 190J, the corrosion rate of sea wave splash is less than or equal to 0.006mm / year, the interface transition layer thickness is less than or equal to 8 μm, and the interface shear strength is greater than or equal to 265MPa.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of building structure steel, and particularly relates to a hot-rolled strip steel for building structure resistant to corrosion in splash zone and a manufacturing method thereof. BACKGROUND

[0002] The ocean is a very harsh and complex corrosion environment. Seawater is a strong electrolyte solution containing high concentrations of chloride ions. Steel facilities as the main structure of marine facilities are prone to electrochemical reactions with the surrounding medium and are severely corroded, greatly reducing the service life of these facilities. Especially in the splash zone, which is the most severe marine environment corrosion area, various facilities are subjected to a series of external factors such as dry-wet alternating, sea spray, sunlight, corrosive components in the atmosphere and oxygen, and the corrosion of materials is particularly serious.

[0003] Investigations have shown that the steel piles of facilities such as marine wharfs and offshore oil platforms in China are severely corroded in this area, which is 3-10 times that of the seawater immersion area. Once severe localized corrosion damage occurs in this area, the carrying capacity of the entire facility will be greatly reduced, the service life will be shortened, production safety will be affected, and even the facility will be prematurely scrapped.

[0004] In the splash zone, due to being in the dry-wet alternating zone, oxygen supply is sufficient, and the corrosion products produced have no protective effect; due to the splashing of seawater, the spray can directly hit the metal surface, causing severe corrosion. Corrosion tests and investigation results show that, under normal circumstances, the average corrosion rate of ordinary carbon steel, low alloy steel and the like in the marine atmosphere is about 0.03-0.08 mm / year, and in the splash zone, it is 0.3-0.5 mm / year. Severe corrosion damage easily occurs in the splash zone, greatly reducing the carrying capacity of the entire steel structure and affecting safe production, shortening the service life and prematurely scrapping the facility.

[0005] According to the above working conditions, industrial pure titanium is selected as the corrosion-resistant layer. Titanium has high chemical activity and is easily reacted with oxygen in the air to form an oxide. The oxide on the surface of titanium metal is dense, stable and has strong self-healing ability. The self-healing ability of titanium oxide mainly refers to the fact that after the titanium oxide film at a certain place on the surface of titanium material is damaged, a new titanium oxide film is rapidly generated to prevent the corrosive medium from further contacting titanium.

[0006] For marine construction steel, in addition to the requirement of corrosion resistance, it also needs to have good mechanical properties, among which the yield ratio and low temperature impact toughness are becoming the focus of attention. The yield ratio is the ratio of yield strength to tensile strength of steel, which reflects the ability of steel to produce strain concentration during plastic deformation. The lower the yield ratio, the more uniform the plastic deformation of steel can be distributed to a wider range. The plastic deformation of steel structure system made of low yield ratio steel can be uniformly distributed to a wider range under the action of earthquake force; while the material with high yield ratio may have strain concentration, which reduces the overall plastic deformation of steel, thus leading to brittle failure of the structure and sudden collapse of the structure at room temperature. Steel will undergo brittle transition at low temperature, and the fracture mode of steel will change from ductile fracture to brittle fracture. The engineering significance is that the structure steel needs to meet the corresponding requirements of low temperature impact performance according to the service environment of the material. The temperature difference of different latitudes of the ocean is large. For example, the temperature near the sea in winter in the Bohai Bay of China can be below-20℃, which requires that the building materials meet the impact performance of-40℃ to ensure that brittle fracture does not occur. If the tensile strength of the material improves while the plasticity and toughness improve, the yield ratio will increase significantly, and it will be difficult to control the low yield ratio.

[0007] Chinese patent CN201210260231.7 discloses a method for preparing titanium-steel-titanium double-sided composite plate. Four titanium plates and three steel plates are stacked in a closed frame made by welding the outermost two steel plates, and a separator made by mixing 1 part by weight of active α-Al2O3 and 1.5 parts by weight of 4% polyvinyl alcohol aqueous solution is added between the titanium plates. Nickel-based alloy is used as the transition layer between the titanium plate and the steel plate. The assembly is heated to 500-630℃ and vacuumized, and the vacuum degree is maintained at 20-200 Pa for 1-2 h. The feature is that the assembly is first welded and then vacuumized. The welding only needs ordinary electric arc welding and submerged arc welding, which has low requirements and low cost compared with vacuum welding, and does not require an additional vacuum chamber. Then the combined blank is rolled in a conventional heating furnace at a rolling temperature of 700-900℃. The outermost steel plate is sealed and vacuumized to block the C in coal gas, and the nickel-based alloy isolation layer is added to prevent the generation of TiC at the interface, thus obtaining a titanium-steel composite plate with a shear strength of 230-260 MPa and an interface bonding rate of 99.6%-100%.

[0008] Chinese patent CN201710769999.X discloses a method for preparing a titanium-steel composite plate, selecting the surfaces of titanium-steel combination blanks in contact with each other, coating the titanium material surface in contact with a high-temperature-resistant anti-carburizing and nitriding isolation coating, and drying at room temperature; after drying treatment is completed, the titanium blanks are aligned and stacked two by two, with a steel blank in between, to complete the combination blank to obtain a composite blank, wherein the thickness of the titanium plate is greater than 2 mm, and the thickness of the steel plate is greater than 5 mm, then the composite blank is sealed and welded around and a certain size of un-welded area is left, the blank is vacuumized to 10 -2 ~10 -3 Pa, and then welded; the plate blank is heated to 500-700°C and rolled, with the first pass reduction exceeding 25%, the last pass reduction not exceeding 15%, the total reduction being 60-70%, and the rolling speed being 0.1-1.0 mm / s. The patent uses a coating with high-temperature anti-permeation protection, which prevents the diffusion and oxidation of other impurity elements at high temperatures and blocks the diffusion of elements such as C and N. The examples use Q235 combined with TA1, and the shear strength of the produced steel plate reaches 176 MPa, 181 MPa, and 182 MPa.

[0009] The above two patents mainly avoid the generation of brittle Ti compounds by adding an additional nickel-based alloy isolation layer between titanium and carbon steel.

[0010] Chinese patent CN201811327623.4 discloses a titanium-steel-titanium composite plate and a method for preparing the same, by fixing a carbon steel between two titanium plates of the same size, warm composite rolling is performed by an irreversible large rolling force warm rolling mill to make the three-layer strip composite into one body, after rolling is completed, the rolled composite plate is subjected to heat treatment operation, including initial annealing at 500-600°C for 20-60 min and recrystallization annealing at 680-700°C for 30-120 min, finally the product is obtained through straightening, leveling, shearing, and shaping, etc. The patent mainly describes a preparation method of a non-heat-rolled composite titanium-steel plate. Since an irreversible rolling mill is used for rolling, only single-pass rolling production can be performed, and heat treatment is also required. The examples mainly relate to a steel strip production method, and the performance after compounding is not mentioned.

[0011] Chinese patent CN201510543767.3 discloses a method for preparing titanium-steel composite plate. The titanium-steel composite plate prepared by the method has high bonding strength. The patent fixes a titanium plate between two carbon steel plates or billets, welds the billets around in a vacuum environment, heats the combined billets to 850-900℃, and controls the open rolling temperature to be above 800℃, the final rolling temperature to be below 700℃, and the single pass deformation amount to be 20-30%, and the total deformation amount to be ≥90%. The method breaks the brittle phase compounds generated at the interface by large reduction rolling to reduce the influence of the brittle phase compounds on the bonding surface, and obtains titanium-steel composite plate with bonding strength greater than 240MPa. The reduction amount and total deformation amount required by the patent are high, which is easy to cause edge weld cracking and destroy the vacuum degree during rolling, and is not suitable for interface bonding.

[0012] Chinese patent CN201610994234.1 discloses a production method of titanium-steel composite plate, and relates to an annealing technology production method of titanium-steel plate. First, titanium plates and steel plates are combined to form a symmetric multi-layer combined billet of steel plate-titanium plate-separator-titanium plate-steel plate. The combined billet is combined by rolling or explosion, and is annealed and pickled by a continuous annealing and pickling line. The combined billet is heated to 500-750℃ to recrystallize the titanium plate, and then heated to 950-1050℃ to recrystallize the steel plate. The patent aims to obtain the properties of the composite material and the base material by two-stage heat treatment. However, the two-stage heat treatment will cause excessive diffusion of titanium, iron and carbon elements, and produce brittle iron-titanium intermetallic compounds and titanium carbide, which will deteriorate the interface shear strength.

[0013] Chinese patent CN201710996925.X discloses a thin composite layer double-sided titanium-steel composite plate and a preparation method thereof. The patent realizes good compounding between titanium and steel by large thickness combination and large reduction rolling technology. The double-sided titanium composite plate is composed of a titanium composite layer, a base layer and a titanium composite layer. The titanium composite layer is made of TA2, and the thickness of the titanium composite layer is 0.2-1mm. The combined billet is placed in the center from top to bottom in the order of cover plate, titanium composite material, carbon steel base material, titanium composite material and cover plate. After vacuumizing in a vacuum chamber, the four around gaps are vacuum electron beam sealed and welded. The vacuum degree is 1.0-4.5×10 -2Pa, the composite blank after the sealing welding treatment is heated to 900-920℃ and kept for a certain time, the holding time is calculated according to 1min / mm x total thickness of the composite blank, the rolling temperature is 880-900℃, the final rolling temperature is above 800℃, air cooling to room temperature, the single pass reduction is ≥15%, and the first three pass reductions are ≥20%, the total reduction is ≥80%, the composite plate obtained after rolling is cut, divided and surface ground to obtain a double-sided titanium steel composite plate, the patent controls the rolling temperature and adopts a large reduction in the way of composite blank surface cleaning treatment and cover isolation of air, so that titanium iron and titanium carbide generated at the composite interface are broken, refined and dispersedly distributed in the composite interface, the distribution state of the compound is improved, the composite quality and performance stability are further ensured, and the shear strength reaches 241MPa.

[0014] Chinese patent CN201710983322.6 discloses a thin composite titanium steel composite plate and a preparation method thereof, which adopts a double-layer structure of titanium and carbon steel composite, the assembly mode and heating process are similar to those of Chinese patent CN201710996925.X, the rolling temperature is 880-900℃, the single pass reduction is 25-30%, the total reduction is ≥85%, while controlling the single pass reduction and the total reduction, the thickness of the titanium steel composite plate is limited to 3-16mm, the final rolling temperature is above 800℃, air cooling to room temperature, the titanium steel composite plate is obtained through surface treatment, the titanium composite layer thickness is ≤1mm, the patent improves the composite quality through the symmetrical assembly mode and the sealing of titanium into the carbon steel plate, the shear strength of the steel plate after rolling reaches above 238MPa, and the composite interface bonding rate is 100%, and the carbon steel layer reaches the national standard requirement of Q420 grade carbon steel.

[0015] The above two patents do not mention the detailed design of the composite layer and the base layer, only the tensile properties and shear strength are described, the pass reduction and total reduction of the composite layer are both high, and the corrosion resistance of the material, the low-temperature impact performance of the base material, the yield ratio and other performance indicators are not controlled, which do not meet the requirements of the steel structure for building structures.

[0016] In summary, the above patents mainly describe the preparation method of the composite steel plate, and the specific embodiments mainly briefly explain the interface shear strength and tensile properties, etc. in terms of performance. The steel structure for the sea-spray zone not only needs to resist corrosion in the sea-spray zone, but also needs to ensure necessary performance requirements of the steel structure, such as the low yield ratio and the corresponding low-temperature impact performance to ensure the safety of the structure. However, the above patents do not design the relevant components and processes for the corrosion rate of the corrosion-resistant layer, the yield ratio and the low-temperature impact, which cannot guarantee that the steel structure for the steel plate in the sea-spray zone can meet the use requirements of the high corrosion-resistant steel structure. SUMMARY

[0017] The application aims to provide a 420MPa-grade hot-rolled strip for building steel structure resistant to splash zone corrosion and a manufacturing method thereof, which can meet the corresponding strength grade requirement of the base layer (carbon steel) and has excellent yield ratio and low-temperature impact toughness without reducing the corrosion resistance of the corrosion-resistant layer; the yield strength of the hot-rolled strip for building structure is greater than or equal to 420MPa, the tensile strength is greater than or equal to 570MPa, the yield ratio is less than or equal to 0.79, the impact energy at-40℃ is greater than or equal to 190J, the splash corrosion resistance is less than or equal to 0.006mm / year, the interface transition layer thickness is less than or equal to 8μm, and the interface shear strength is greater than or equal to 265MPa; the hot-rolled strip for building structure can meet the corrosion resistance requirement in the splash zone environment, has excellent mechanical properties and high economy, and can be applied to steel structural members such as steel piles of facilities such as seaports and offshore oil platforms.

[0018] To achieve the above-mentioned purpose, the technical scheme of the application is as follows:

[0019] The application adopts low-carbon micro-alloying component design, realizes excellent combination of titanium and carbon steel without adding metal isolation layer, controls the interface transition layer thickness, and meets the corresponding strength grade requirement of the base layer (carbon steel) without reducing the corrosion resistance of the corrosion-resistant layer.

[0020] Specifically, the 420MPa-grade hot-rolled strip for building structure resistant to splash zone corrosion comprises a base layer, a corrosion-resistant layer, and an interface transition layer between the base layer and the corrosion-resistant layer.

[0021] The chemical component of the base layer is as follows: C 0.03-0.15%, Si 0.15-0.35%, Mn 1.0-1.5%, P 0.0005-0.003%, S 0.0005-0.01%, Cr 0.08-0.45%, Ni 0.08-0.60%, Cu 0.05-0.2%, Al 0.02-0.05%, Ti 0.009-0.016%, Nb 0.03-0.06%, N 0.0005-0.005%, V 0.08-0.25%, and the balance of Fe and other inevitable impurities.

[0022] The corrosion-resistant layer is made of industrial pure titanium.

[0023] The yield strength of the hot-rolled strip for building structure is greater than or equal to 420MPa, the tensile strength is greater than or equal to 570MPa, the yield ratio is less than or equal to 0.75, the impact energy at-40℃ is greater than or equal to 190J, the splash corrosion resistance is less than or equal to 0.006mm / year, the interface transition layer thickness is less than or equal to 8μm, and the interface shear strength is greater than or equal to 265MPa.

[0024] Preferably, the base layer chemical composition also satisfies the following relationship:

[0025] 0.20%≤Cu+Ni≤0.75%;

[0026] 2(C+N)≤Ti+Nb+Cr+V≤0.60%.

[0027] Further, the balance of the base layer composition is Fe and other unavoidable impurities.

[0028] The microstructure of the base layer is ferrite + bainite + martensite, with a bainite + martensite content of 5-15%, and an average ferrite grain size of ≥8.5 grade.

[0029] The base layer has a yield strength of ≥420 MPa, a tensile strength of ≥570 MPa, a yield strength ratio of ≤0.75, and an impact energy at -40℃ of ≥190 J.

[0030] Preferably, the corrosion-resistant layer is made of TA1, TA2, TA3 or TA4.

[0031] The microstructure of the corrosion-resistant layer is single, equiaxed α-Ti.

[0032] The corrosion-resistant layer has a sea-spray corrosion rate of ≤0.006 mm / year.

[0033] Preferably, the interface transition layer achieves 100% metallurgical bonding, atomic-level coherent, an interface transition layer thickness of ≤8 μm, and an interface shear strength of ≥265 MPa.

[0034] Preferably, the interface transition layer has fine grains with an average grain size of 15-40 μm, and contains less than 120 nm (Ti, Nb)C precipitated particles.

[0035] Preferably, the hot-rolled strip steel for building structures has a thickness of 1.0-20 mm.

[0036] In the base layer composition design of the hot-rolled strip steel for building steel structures according to the present application:

[0037] C: C plays a role of solid solution strengthening in steel, and can significantly improve the strength of the steel. However, too high a content of C is not conducive to the weldability and toughness, and more importantly, too high a content of C will diffuse to the composite interface, forming a large amount of TiC hard phase in the form of large particles in the interface transition layer, thereby reducing the strength of the composite interface. In order to ensure the shear strength of the interface, a low content of C is used. The change in the content of C has a smaller effect on the yield strength of the steel than on the tensile strength. Under the premise of ensuring the forming and weldability of the product, appropriately increasing the content of C is beneficial to reducing the yield strength ratio of the steel. Based on this, the content of C in the base layer composition according to the present application is controlled to be 0.03-0.15%.

[0038] Si: Si element can effectively deoxidize in steel to improve the purity of steel. In addition, Si element can play a solid solution strengthening effect in steel, which can improve the strength and hardness of steel material, but Si element is not conducive to the welding performance of the material. Therefore, the Si content is controlled at 0.10-0.30% in the base layer composition of the application.

[0039] Mn: Mn is the cheapest strengthening matrix element, which can reduce the austenite transformation temperature, delay the pearlite transformation, refine the ferrite grain, and improve the strength of the steel. At the same time, Mn can also eliminate the effect of S on steel. However, too high Mn content can easily cause segregation band and martensite organization, which is not conducive to the toughness of the steel. Therefore, the Mn content is controlled at 1.0%-1.5% in the base layer composition of the application.

[0040] Al: Al is mainly added to steel as a deoxidizing element to ensure that the O content in the steel is as low as possible. After deoxidization, the excess Al combines with N element in the steel to form AlN precipitates. AlN hinders the growth of austenite grains during heating, refines the austenite grains, and improves the strength and toughness of the matrix. At the same time, the formation of AlN fixes part of the N in the matrix, reduces the diffusion of interstitial atoms N in the carbon steel base layer to the interface transition layer to form hard TiN, and deteriorates the interface shear strength of the composite plate. At the same time, the addition amount of Ti and Nb can be reduced to reduce the total cost. Therefore, the Al content is controlled at 0.015-0.03% in the base layer composition of the application.

[0041] Ti: Ti forms stable TiN or Ti(N,C) at high temperature, which plays a role of fixing C and N, prevents the diffusion of interstitial C and N atoms in the carbon steel base layer to the interface, and forms hard TiN or Ti(N,C) precipitates in the interface transition layer to obtain a composite plate with high interface shear strength. At the same time, TiN hinders the growth of austenite during heating, refines the austenite grains, and can improve the strength and toughness of the matrix. In subsequent welding, especially in the heat affected zone (HAZ) close to the weld fusion boundary, the growth of austenite grains is inhibited, thereby improving the toughness of the welding HAZ and meeting the needs of large welding heat input process. To improve the strength of the low-carbon matrix, reduce the diffusion of C and N to the interface, and obtain a composite plate with high interface shear strength. Therefore, the Ti content is controlled at 0.009-0.016% in the base layer composition of the application.

[0042] Nb: Nb exists in the form of solid solution Nb and Nb(C,N) in the steel, and plays a role of solid solution drag and precipitate pinning in the process of recrystallization. A small amount of Nb is added in the base carbon steel mainly to increase the recrystallization temperature, so that the base carbon steel is rolled in the recrystallization and non-recrystallization zone after grain refinement, which is beneficial to improve the low temperature impact toughness of the base carbon steel. Due to the effect of Nb(C,N) precipitate phase, the original austenite grain will be finer, thereby promoting the formation of finer recrystallized grains, obtaining the ideal combination of high strength and high toughness, at the same time, Nb can fix the gap C, N atoms in the matrix, reduce the diffusion of C, N to the interface, and obtain high interface shear strength clad plate. Based on this, the content of Nb in the base layer described in the application is controlled at 0.030-0.06%.

[0043] Cu: Cu plays a role of solid solution strengthening, and with the increase of Cu content, the room temperature impact toughness of the steel is slightly improved, therefore, the content of Cu in the base layer composition described in the application is controlled at 0.050-0.20%.

[0044] N: N element can generate second phase particles with Ti and Al, refine austenite grains, and improve the strength and toughness of the matrix. However, when the content of N is too high, the amount of TiN generated is too large and the particles are too coarse, which will affect the plasticity and toughness of the base carbon steel of the application. Based on this, the content of N in the base layer described in the application is controlled at 0.0005-0.005%.

[0045] Ni: Ni is an element for stabilizing austenite, and has a certain effect on improving toughness and strength. Adding Ni in the steel can greatly improve the low temperature impact toughness of the steel. However, nickel is expensive, and adding too much will increase the cost of the clad plate. Based on this, an appropriate amount of Ni is added in the base layer composition described in the application, and the content of Ni is controlled at 0.08-0.60%.

[0046] Cr: Cr is a strong carbonization forming element, and has a small diffusion speed in austenite, and at the same time, hinders the diffusion of C element. Fine carbides are formed in the low temperature process, which plays a role of precipitate strengthening, at the same time, can fix the gap C, N atoms in the matrix, reduce the diffusion of C, N to the interface, and obtain high interface shear strength clad plate. Cr can increase the strength of the matrix in the steel while reducing the toughness, in order to obtain the best match of strength and toughness, the content of Cr in the base layer composition described in the application is controlled at 0.08-0.45%.

[0047] V: V is a strong carbonitride forming element, and when Ti and Nb are added in the steel, fine composite carbonitrides can be formed, and the precipitation temperature range is wide, which can effectively inhibit the austenite grain growth and recrystallization process, and improve the strength and toughness of the base carbon steel. At the same time, the carbonitride precipitation temperature of V is low, and the precipitation during phase transformation can effectively organize the growth of ferrite grains and strengthen the ferrite matrix strength. Therefore, in the base composition of the present application, an appropriate amount of V is added, and the V content is controlled to be 0.08-0.25%.

[0048] S, P, both are inevitable impurity elements, and the lower the content is, the better. Considering the actual steelmaking level of the steel plant, the S and P contents in the present application are controlled as follows: S≤0.010%; P≤0.003%;

[0049] The corrosion-resistant layer of the present application uses industrial pure titanium, preferably TA1, TA2, TA3 and TA4, and the composition meets the GB / T3620.1-2016 standard of "Titanium and Titanium Alloy Grade and Chemical Composition".

[0050] In addition, in the base layer chemical composition design of the hot-rolled strip steel for building steel structure of the present application:

[0051] Because Cu and Ni can improve the toughness of the base layer, and the combined addition effect is particularly significant, at the same time, the addition of Ni element can reduce the diffusion rate of C in the steel, and reduce the diffusion of C to the interface, therefore, the present application controls 0.20%≤Cu+Ni≤0.75%, and the interface transition layer can be controlled within 8μm.

[0052] Because Ti, Nb, Cr and V are all strong carbonitride forming elements, corresponding carbonitrides can be formed in the base carbon steel, which can fix the gap atoms in the base layer, hinder the diffusion of C, N gap atoms to the interface, and form large particles, aggregated carbonitrides in the interface transition layer, so that the interface transition layer can be controlled within 8μm, thereby improving the interface shear strength. At the same time, Ti, Nb and Cr can refine the grains of the base carbon steel at different stages of hot rolling process, and improve the toughness. Therefore, the present application controls: 2(C+N)≤Ti+Nb+Cr+V≤0.60%.

[0053] The manufacturing method of the 420MPa grade hot-rolled strip steel for building structure resistant to corrosion in sea wave splash zone of the present application comprises the following steps:

[0054] 1) Smelting and casting

[0055] According to the compositions of the above base layer and corrosion-resistant layer, the blanks are respectively smelted and cast;

[0056] 2) Grouping

[0057] The surface of the base layer and the corrosion-resistant layer blank is polished, and the surrounding of the blank is welded to form a composite blank; the combined surface after welding is sealed and vacuumized;

[0058] 3) heating

[0059] The composite blank is heated to 900-1000℃;

[0060] 4) rolling

[0061] The rough rolling temperature is controlled above 860℃, the finish rolling temperature is controlled at 780-850℃, the pass reduction rate is controlled at 5-20%, and the cumulative reduction rate is ≥ 88%;

[0062] 5) cooling

[0063] After the first stage, the steel strip is cooled to 700-750℃ at a cooling rate of ≤10℃ / s after leaving the rolling stand, and then in the second stage, the steel strip is cooled to 350-450℃ at a cooling rate of 30-50℃ / s and then coiled.

[0064] Preferably, the thickness of the corrosion-resistant layer in step 1) is 1-10% of the total thickness of the composite blank.

[0065] Preferably, the pass reduction rate used in step 4) rolling is 10-15%.

[0066] In the manufacturing method described in the present application:

[0067] 1) smelting: P and S elements can deteriorate the fracture toughness of the steel, so low P and low S control should be performed during smelting to improve the quality of the billet. Clean steel production technology is adopted to reduce the gas and inclusion content in the steel and improve the comprehensive performance of the steel, especially the resistance to lamellar tearing.

[0068] 2) grouping; the thickness of the corrosion-resistant layer is prepared according to 1-10% of the total thickness of the composite blank. The corrosion-resistant layer and the base layer carbon steel blank are pretreated, and the surrounding of the steel blank is welded and sealed, and the combined surface after welding is vacuumized. Vacuum treatment protects the surface of the corrosion-resistant layer from oxidation, which is also an important condition for ensuring the corrosion resistance of the corrosion-resistant layer in the splash area, and at the same time, it can ensure that the corrosion-resistant layer does not crack and break during the subsequent large cumulative reduction rate deformation process.

[0069] 2) grouping; the thickness of the corrosion-resistant layer is prepared according to 1-10% of the total thickness of the composite blank. The corrosion-resistant layer and the base layer carbon steel blank are pretreated, and the surrounding of the steel blank is welded and sealed, and the combined surface after welding is vacuumized. Vacuum treatment protects the surface of the corrosion-resistant layer from oxidation, which is also an important condition for ensuring the corrosion resistance of the corrosion-resistant layer in the splash area, and at the same time, it can ensure that the corrosion-resistant layer does not crack and break during the subsequent large cumulative reduction rate deformation process.

[0070] 3) Heating: for single carbon steel, the slab heating temperature is generally controlled at 1000-1250℃, which is beneficial to the dissolution and sufficient diffusion of precipitates in the steel, promotes the slab element homogenization, and plays a strengthening role of micro-alloy elements in the steel; for single industrial pure titanium plate, the heating temperature is generally controlled at 850-1000℃, and a high heating temperature will produce β phase transformation, and the β phase will grow rapidly, which will deteriorate the performance of industrial pure titanium. Because the high heating temperature will also make the elements diffuse sufficiently, which promotes the subsequent realization of 100% metallurgical bonding of the interface; but the high heating temperature will increase the tendency of austenite grain coarsening, increase the difficulty of subsequent controlled rolling, and most importantly, it will accelerate the diffusion of C, N, Ti, Fe to the interface, form a thick brittle precipitate and intermetallic compound at the interface, form a thick interface transition layer, and deteriorate the interface shear strength. Preferably, a relatively low heating temperature is used for traditional carbon steel production, and the heating temperature is set to 900-1000℃.

[0071] 4) Rolling: the rough rolling temperature is controlled above 860℃. In the high temperature rough rolling area, a large reduction rate is rolled to make the structure fully recrystallize, refine the grain, and improve the strength and toughness of the material; controlled rolling is carried out in the non-recrystallization zone of the finish rolling, and no austenite recrystallization occurs in this stage. Through reasonable reduction rate and finish rolling temperature, deformation energy and dislocation are accumulated to form high-density deformation bands inside the austenite grains, increase the ferrite phase nucleation points, further refine the grain size after the phase transformation of the base body, and improve the strength and toughness of the material. At the same time, in this stage, deformation induces the precipitation of Nb, Ti, Cr and V carbonitride, which improves the strength of the base body, suppresses the diffusion of C to the interface, and avoids the formation of too thick TiC at the interface to deteriorate the interface shear strength. The pass reduction rate is ensured at 5-20%, and the cumulative reduction rate is ≥88%; preferably, the finish rolling temperature is controlled at 780-850℃, which ensures the corrosion performance of titanium, avoids the base layer from being rolled in the two-phase zone, and obtains ferrite with an average grain size of ≥8.5 grade and bainite+martensite structure with a content of 5-15%.

[0072] 5) cooling: by controlling cooling through open cooling, final cooling and cooling speed, the control of the microstructure type, microstructure size and each content after rolling is realized. If the cooling speed is too fast, a large amount of bainite and martensite microstructure will be formed, which is low toughness and high yield ratio microstructure, and is not conducive to the performance of the steel plate. If the cooling speed is too slow, a large amount of coarse ferrite microstructure will be formed, which is beneficial to the crack propagation and causes the impact performance to decrease, so the cooling speed should be reasonably controlled; the control of the finish rolling temperature can avoid the abnormal coarse microstructure caused by rolling in the two-phase region; at the same time, the rolling can be quickly cooled to the phase transition temperature to further inhibit the growth of the microstructure, the strength and low temperature impact toughness of the material are improved by refining the grain, and more than 5% of the hard phase microstructure is formed to ensure the strength. Preferably, a two-stage cooling mode is adopted, the cooling speed is ≤10℃ / s after the rolling machine rack, and then the cooling is cooled to the ferrite phase transition temperature, and then the cooling speed is quickly cooled to 350-450℃ at 30-50℃ / s to obtain a small amount of bainite and martensite microstructure, and the microstructure is recovered during the coiling process, and the V carbonitride is precipitated to ensure that the base layer has a low yield ratio and high low temperature impact toughness.

[0073] Preferably, when the corrosion-resistant layer is too thick, the mechanical properties of the material and the production cost will be affected; and when the corrosion-resistant layer is too thin, the corrosion resistance and service life of the material will be reduced. Therefore, the ratio of the corrosion-resistant layer to the total thickness of the composite blank in the above blanking process is preferably 1-10%.

[0074] The present application forms a corrosion-resistant layer on the surface of the base layer, i.e. the carbon steel strip, by rolling process, and finally forms a steel strip with good mechanical properties and high economic efficiency, which can be effectively applied to steel structural members used in sea-spray area environment.

[0075] Compared with the prior art, the present application has the following beneficial effects:

[0076] The present application adopts low-carbon micro-alloying composition design, realizes the excellent combination of titanium and carbon steel under the condition of no metal isolation layer, controls the thickness of the interface transition layer, and the mechanical properties of the base layer (carbon steel) can meet the corresponding strength level requirements without reducing the corrosion resistance of the corrosion-resistant layer itself, and the base layer has excellent yield ratio and low temperature impact toughness.

[0077] The present application adopts low-C micro-alloy design, reduces the formation of TiC compounds in the interface transition layer by reducing C, and forms carbonitride in the base layer to hinder grain growth and improve the low-temperature impact toughness of the base layer. Meanwhile, the addition of micro-alloy elements and the reasonable rolling and cooling process solve the problem of low strength of the material under low-carbon condition, with yield strength ≥420 MPa, tensile strength ≥570 MPa, and ensuring that the material yield strength ratio ≤0.79 and the-40℃ impact energy ≥190J, which are all higher than the performance requirements in the national standard GB / T19879-2015 "Building Structure Steel Plate".

[0078] Chinese patent CN201210260231.7 does not specify the heating temperature, and adds a layer of nickel plate as a separation layer between the composite layer and the base layer to prevent the formation of TiC at the interface, and the interface bonding rate of the titanium steel composite plate obtained is 99.6-100%. The present application specifies that the composite blank heating temperature is 900-1000℃, reduces the addition of Ni through low-carbon and micro-alloy design, reduces the production cost, and at the same time, through heating, rolling and other process optimization, a certain thickness of transition layer organization can be formed, and the formation of brittle phase TiC in the interface transition layer is reduced, and complete metallurgical bonding with an interface bonding rate of 100% is realized.

[0079] Chinese patent CN201710769999.X has a slab heating temperature of 500-700℃ and a total reduction of area of 60-70%, and the interface shear strength of the produced strip steel is at most 182MPa. The present application fully considers the influence of the high-temperature phase transition of the corrosion-resistant layer of industrial pure titanium on corrosion resistance and the strength and toughness control of the base layer carbon steel. Combined with low-carbon micro-alloy design, through overall design of the processing technology, the heating temperature of the composite blank is set to 900-1000℃, at which temperature the corrosion-resistant layer does not undergo phase transition, and the precipitates in the base layer carbon steel are fully dissolved, which plays a role in refining the base layer grains and improving the strength and toughness of the base layer during controlled rolling, combined with a cumulative reduction of area ≥85%, so that the brittle phase of the interface transition layer is broken and the interface shear strength is improved.

[0080] The above two patents mainly avoid the formation of brittle Ti compounds by adding an additional nickel-based alloy separation layer between titanium and carbon steel, while the present application does not add a separation layer through component and process design, and the base layer carbon steel material is also significantly different from the above two patents.

[0081] Under the process conditions of the present application, the corrosion resistance of the corrosion-resistant layer of industrial pure titanium is ensured, and the mechanical properties of the base layer are ensured, solving the problem that the traditional processing process windows of titanium and carbon steel are too different to be considered together. At the same time, the interface transition layer with a thickness of not greater than 10 μm is formed by controlling the full diffusion of the elements of the base layer and the corrosion-resistant layer, the microstructure of the layer has fine grains with an average grain size of 15-40 μm, and contains less than 120 nm (Ti, Nb) C precipitated particles, which strengthens the interface bonding performance and ensures that the interface shear strength is greater than or equal to 265 MPa, which is higher than the interface shear strength of 182 MPa of Chinese patent CN201710769999.X.

[0082] Chinese patent CN201811327623.4 realizes compounding by warm rolling, and then needs to perform two-stage heat treatment operations, including primary annealing at 500-600 °C for 20-60 min and recrystallization annealing at 680-700 °C for 30-120 min, which is a preparation method of a non-hot-rolled composite titanium strip, and is completely different from the manufacturing method of the present application.

[0083] Chinese patent CN201510543767.3 requires a heating temperature of 850-900 °C, a final rolling temperature of less than 700 °C, and a single-pass deformation amount of 20-30 %, and the total deformation amount is greater than or equal to 90 %, and the shear strength of the titanium-steel composite plate is greater than 240 MPa. The pass reduction amount and the total deformation amount required by the patent are very high, and the edge weld is prone to cracking during rolling, which destroys the vacuum degree, and it is not easy to compound, and the rolling stability is poor. The single-pass deformation amount of the present application is controlled to be 5-20 %, which can effectively control the weld from cracking during rolling, ensure the vacuum degree inside the slab, and improve the interface shear strength, the rolling stability and the success rate.

[0084] Chinese patent CN201610994234.1 is an annealing technology production method for a titanium steel plate, which first forms a symmetrical multi-layer combined blank of steel plate-titanium plate-separator-titanium plate-steel plate by combining titanium plates and steel plates, and then performs compounding by rolling compounding or explosive compounding. The combined blank is annealed and pickled by a continuous annealing and pickling line, is heated to 500-750 °C first to recrystallize the core titanium plate, and is then heated to 950-1050 °C to recrystallize the base steel plate. The rolling process and the corrosion and structural properties of the obtained steel plate are not specifically clear. The present application is obviously different from the manufacturing process of the present application, and the two-stage heat treatment will cause excessive diffusion of titanium, iron and carbon elements, produce brittle intermetallic compounds of iron and titanium and titanium carbide, and deteriorate the interface shear strength.

[0085] The corrosion-resistant layer material of Chinese patent CN201710996925.X is TA2, wherein the titanium complex layer thickness is 0.2-1 mm, heating to 900-920 DEG C, open rolling temperature is 880-900 DEG C, the final rolling temperature is 800 DEG C or above, air cooling to room temperature, and the shear strength reaches 241 MPa. The heating temperature is 900-1000 DEG C, the finish rolling temperature is 750 DEG C-850 DEG C, and the two-stage cooling mode is used for cooling. Under this process, the TA1, TA2, TA3 and TA4 can be used as the corrosion-resistant layer, the corrosion-resistant layer accounts for 0.5-20% of the total thickness of the composite blank, and the interface shear strength is greater than or equal to 265 MPa.

[0086] The group blank mode and heating process of Chinese patent CN201710983322.6 are similar to those of Chinese patent CN201710996925.X, the single pass reduction rate is 25-30%, the total reduction rate is greater than or equal to 85%, while controlling the single pass reduction rate and the total reduction rate, the thickness of the titanium steel composite plate is limited to 3-16 mm, the final rolling temperature is 800 DEG C or above, air cooling to room temperature, and the titanium steel composite plate is obtained through surface treatment, and the titanium complex layer thickness is less than or equal to 1 mm. The single pass reduction rate is 5-20%, the rolling stability is controlled, the two-stage cooling mode is used, the material organization type is ensured, and thus the yield ratio and toughness are controlled. There are also obvious differences in the corrosion-resistant layer thickness and the total thickness of the composite strip steel.

[0087] The 420MPa-grade hot-rolled strip steel for building structure resistant to corrosion in sea-spray area can solve the essential problems of stainless steel or carbon steel used in the sea-spray area environment, can be effectively applied to the manufacture of steel structural parts used in the sea-spray area environment, such as the steel structural parts of facilities such as seaport wharfs and offshore oil platforms in the sea-spray area, can meet the requirements of these components for corrosion resistance and mechanical properties in the sea-spray area, greatly improves the applicability, safety and durability of these components, and has great economic and social benefits. BRIEF DESCRIPTION OF DRAWINGS

[0088] Figure 1 It is a schematic diagram of the interlayer structure of the 420MPa-grade hot-rolled strip steel for building structure resistant to corrosion in sea-spray area.

[0089] Figure 2 It is another schematic diagram of the interlayer structure of the 420MPa-grade hot-rolled strip steel for building structure resistant to corrosion in sea-spray area.

[0090] Figure 3 It is a microstructure photo of the corrosion-resistant layer of embodiment 3.

[0091] Figure 4A scanning image of the interface transition layer for the base layer and the corrosion-resistant layer of Example 3 of the present application.

[0092] Figure 5 A microstructure photograph of the base layer of Example 3 of the present application. DETAILED DESCRIPTION

[0093] The technical solutions of the present application will be further described in detail below in combination with examples and drawings. It should be clear that the following examples are only used to describe the specific embodiments of the present application and do not constitute any limitation on the protection scope of the present application.

[0094] Reference is made to Figure 1 , Figure 2 , Figure 4 , which shows a schematic diagram of two interlayer structures of the hot-rolled strip steel for building structure according to the present application, wherein 1 is the base layer, 2 is the corrosion-resistant layer, and 3 is the interface transition layer.

[0095] The composition of the base layer of the hot-rolled strip steel (composite steel plate) embodiment of the present application is shown in Table 1, and the remaining amount of the composition is Fe and unavoidable impurities. Table 2 shows the manufacturing process parameters of the composite steel plate embodiment of the present application. Table 3 shows the metallographic structure and mechanical properties of the base layer and the corrosion-resistant layer in the composite steel plate of the example and the comparative example, and the thickness of the interface transition layer.

[0096] The yield strength and tensile strength of the composite steel plate are measured in accordance with GB / T 6396-2008 "Method for Mechanical and Technological Properties of Composite Steel Plate" and GB / T 228-2010 "Metallic Materials Tensile Test Method at Room Temperature".

[0097] The impact energy KV2 / J (longitudinal) of the base layer carbon steel at -40℃ is measured in accordance with GB / T 6396-2008 "Method for Mechanical and Technological Properties of Composite Steel Plate" and GB / T 229-2020 "Metallic Materials Charpy Pendulum Impact Test Method".

[0098] The grain size rating is as follows: the ferrite structure in stainless steel and carbon steel is rated according to GB / T 6394-2017 "Metallic Material Average Grain Size Determination Method" using the intercept method.

[0099] The comparative example is prepared using the above steps which are basically the same as the example of the present application, except that the composition of the base layer carbon steel and certain process parameters used during the rolling or cooling steps do not meet the requirements of the present application.

[0100] The metallographic structure of the corrosion-resistant layer of Example 3 is shown in Figure 3 , which shows single, equiaxed α-Ti with an average grain size of 103.4 um.

[0101] The interface transition layer of Example 3 is shown inFigure 4 The thickness of the interface transition layer 3 is 7.3 μm, wherein the discontinuous fine particles are TiC with a size less than 120 nm.

[0102] The microstructure of the base layer of Example 3 is shown in Figure 5 The microstructure of the base layer of Example 3 is shown in

[0103] Table 4 shows the corrosion of the hanging samples of the composite steel plates of Examples 1-8 and Comparative Examples 1-4 after 6 months in the splash zone of the South China Sea. The observation results show that the corrosion rates of the Examples and Comparative Examples are all less than or equal to 0.006 mm / year, except for Comparative Example 4.

[0104] Comparative Examples 1-4 do not meet the requirements of the component design, the hot working process conditions, and thus the properties of the composite steel plates cannot meet the use requirements (the property parameters are not within the range defined in the application). Among them:

[0105] Comparative Example 1 has a pass reduction rate and a cumulative reduction rate that are not within the range defined in the application, resulting in coarse grains, and the yield strength and impact energy cannot meet the requirements.

[0106] Comparative Example 2 has a Cu+Ni addition amount less than 0.2%, and thus the impact performance cannot meet the requirements.

[0107] Comparative Example 3 has a coiling temperature that is not within the range defined in the application, and the yield strength ratio and impact performance cannot meet the requirements.

[0108] Comparative Example 4 does not add V element, and the cooling speed after exiting the rack and the second stage cooling speed are not within the range defined in the application, and thus the necessary precipitation strengthening effect is lacking, and the yield strength and tensile strength cannot meet the requirements.

[0109] Comparative Example 5 has a heating temperature and a finish rolling temperature that are not within the range defined in the application, and the transition layer thickness is increased, and the shear strength cannot meet the requirements.

[0110] Through the preparation method of the application, especially the control of the heating, rolling and cooling processes, the base layer in the strip steel exhibits a low yield strength ratio and good low-temperature impact toughness, and the cladding layer has excellent corrosion resistance and high bonding strength. The yield strength is 430-508 MPa, the tensile strength is 577-645 MPa, the yield strength ratio is less than or equal to 0.79, the impact energy at -40°C is more than 190 J, and the interface shear strength is more than 265 MPa.

[0111] It should be noted that all technical features described in the present application can be freely combined or integrated in any manner unless they contradict each other. Various modifications and changes can be made to the present application without departing from the scope of the present application, which will be apparent to those skilled in the art. For example, features shown or described as part of one embodiment can be used with another embodiment to produce yet another embodiment. Accordingly, the present application is intended to embrace all such modifications and changes and follows the scope of the appended claims and their equivalents.

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Claims

1. A hot-rolled strip steel for building structure with 420 MPa grade resistance to sea-spray corrosion, comprising a base layer, a corrosion-resistant layer and an interface transition layer between the base layer and the corrosion-resistant layer; the base layer has the following chemical composition in mass percentage: C 0.03-0.15%, Si 0.15-0.35%, Mn 1.0-1.5%, P 0.0005-0.003%, S 0.0005-0.01%, Cr 0.08-0.45%, Ni 0.08-0.60%, Cu 0.05-0.2%, Al 0.02-0.05%, Ti 0.009-0.016%, Nb 0.03-0.06%, N 0.0005-0.005%, V 0.08-0.25%, and the balance of Fe and other inevitable impurities; the corrosion-resistant layer is made of industrial pure titanium; the hot-rolled strip steel for building structure has a yield strength of ≥ 420 MPa, a tensile strength of ≥ 570 MPa, a yield strength ratio of ≤ 0.79, an impact energy at -40 ℃ of ≥ 190 J, a sea-spray corrosion resistance rate of ≤ 0.006 mm / year, an interface transition layer thickness of ≤ 8 μm, and an interface shear strength of ≥ 265 MPa.

2. The hot-rolled steel strip for a building structure according to claim 1, characterized by, the base layer chemical composition also satisfies the following relationship: 0.20%≤ Cu+Ni ≤ 0.75%; 2(C+N)≤ Ti+Nb+Cr+V ≤ 0.60%.

3. The hot-rolled strip for a building structure according to claim 1 or 2, characterized by, the industrial pure titanium is TA1, TA2, TA3 or TA4.

4. The hot-rolled strip for a building structure according to claim 1 or 2, characterized by the base layer has a microstructure of ferrite + bainite + martensite, and the bainite + martensite content is 5-15%, and the average ferrite grain size is ≥ 8.5 grade.

5. The hot-rolled strip for a building structure according to claim 1 or 2, characterized by the base layer has a yield strength of ≥ 420 MPa, a tensile strength of ≥ 570 MPa, a yield strength ratio of ≤ 0.79, and an impact energy at -40 ℃ of ≥ 190 J.

6. The hot-rolled steel strip for a building structure according to claim 4, characterized by, the base layer has a yield strength of ≥ 420 MPa, a tensile strength of ≥ 570 MPa, a yield strength ratio of ≤ 0.79, and an impact energy at -40 ℃ of ≥ 190 J.

7. The hot-rolled strip steel for a building structure according to claim 1, wherein the microstructure of the corrosion-resistant layer is single and equiaxed α-Ti.

8. The hot-rolled steel strip for a building structure according to claim 1 or 7, characterized in that, the sea-spray corrosion resistance rate of the corrosion-resistant layer is ≤ 0.006 mm / year.

9. The hot-rolled strip steel for a building structure according to claim 1, wherein the interface transition layer realizes 100% metallurgical bonding, atomic high coherence, an interface transition layer thickness of ≤ 8 μm, a small grain size of the layer, an average grain size of 15-40 μm, and contains less than 120 nm (Ti, Nb)C precipitated particles, and an interface shear strength of ≥ 265 MPa.

10. The hot-rolled steel strip for a building structure according to claim 1, characterized by, the hot-rolled strip steel for building structure has a thickness of 1.0-20 mm.

11. The method of producing a hot-rolled strip steel for a building structure of 420 MPa grade resistant to corrosion in a splash zone of sea waves according to any one of claims 1 to 10, characterized by, comprising the following steps: 1) smelting and casting smelting and casting the base layer and the corrosion-resistant layer according to claim 1 or 2 into blanks respectively; 2) blank assembly surface grinding and polishing the base layer and the corrosion-resistant layer blanks, and welding and sealing the surrounding surface of the blanks to form a composite blank, and performing vacuum treatment on the welded and sealed joint surface; 3) heating heating the composite blank to 900-1000 ℃; 4) rolling controlling the rough rolling temperature to be ≥ 860 ℃, the finish rolling temperature to be 780-850 ℃, and the pass reduction rate to be 5-20%; and the cumulative reduction rate is ≥ 88%; 5) cooling After the first stage steel strip leaves the rolling stand, it is cooled to 700-750°C at a cooling rate of ≤10°C / s, and then in the second stage it is cooled to 350-450°C at a cooling rate of 30-50°C / s and coiled.

12. The production method according to claim 11, wherein Step 1) the corrosion resistant layer has a thickness of 1-10% of the composite blank thickness.

13. The production method according to claim 11, wherein In step 4) the pass reduction is 10-15%.

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